Metal injection moulding method

ABSTRACT

A method for forming an article by metal injection moudling of aluminium or an aluminium alloy. The method comprises the steps of forming a mixture containing an aluminium powder or an aluminum alloy powder or both and optionally ceramic particles, a binder, and a sintering aid comprising a low melting point metal. The mixture is injection moulded and the binder is removed to form a green body. The green body is sintered. The sintering is conducted in an atmosphere containing nitrogen and in the presence of an oxygen getter.

FIELD OF THE INVENTION

The present invention relates to a metal injection moulding method.

BACKGROUND TO THE INVENTION

Metal injection moulding involves the mixing of powder metal with abinder to form a feedstock. This mixture is then injection moulded usinginjection moulding equipment that is similar to that used in theplastics industry. This forms a “green body”. The green body hassufficient rigidity and strength to enable handling. The green body isthen further treated to remove the binder and to sinter the metal powderparticles to form the final article.

The binder typically comprises one or more thermoplastic compounds,plasticisers and other organic material. Ideally, the binder is moltenor liquid at the injection moulding temperature but solidifies in themould when the green body is cooled. The feedstock may be converted intosolid pellets, for example by granulation. These pellets may be storedand fed into the injection moulding machine at a later time.

Typical injection moulding equipment includes a heated screw or extruderhaving a nozzle through which the mixture is extruded into the diecavity. The extruder is heated to ensure that the binder is in liquidform and the nozzle temperature is typically carefully controlled toensure constant conditions. Desirably, the temperature of the die isalso controlled so that the temperature is low enough to ensure that thegreen body is rigid when it is removed from the die.

As the binder can occupy a substantial volume fraction of the greenbody, the green body will be larger than the final article.

Further processing of the green body involves removing the binder andsintering. The binder may be completely removed before sintering.Alternatively, the binder may be partly removed before the sinteringstep, with complete removal of the binder being achieved during thesintering step.

Removal of the binder may take place by using a solvent to dissolve thebinder or by heating the green body to cause the binder to melt,decompose and/or evaporate. A combination of solvent removal and thermalremoval may also be used.

The sintering step involves heating the body to cause the separate metalparticles to metallurgically bond together. Sintering in the productionof metal injection moulded parts is generally similar to sintering usedin the production of traditional powder metal parts. Non-oxidisingatmospheres are typically used during the sintering step in order toavoid oxidation of the metal. During sintering in metal injectionmoulding methods, the very porous body remaining after removal of thebinder densifies and shrinks. The sintering temperature and temperaturedistribution will typically be closely controlled in order to retain theshape of the article during sintering and to prevent distortion of thearticle. In this fashion, net shape articles can be recovered from thesintering step:

Metal injection moulding is suitable for producing articles from almostany metal that can be produced in a suitable powder form. However,aluminium is difficult to use in metal injection moulding because theadherent aluminium oxide film that is always present on the surface ofparticles of aluminium or aluminium alloy inhibits sintering.

U.S. Pat. No. 6,761,852, assigned to Advanced Materials Technologies PteLtd, describes a metal injection moulding process for forming objectsfrom aluminium and its alloys. In this process, a powder of aluminium oran aluminium alloy is mixed with a powder containing a material that issaid to form a eutectic with aluminium oxide, such as silicon carbide ora metallic fluoride. This mixed powder is then mixed with a binder,injection moulded, subject to removal of the binder, and sintered.

In the process of U.S. Pat. No. 6,761,852, the silicon carbide or metalfluoride is said to form a eutectic mixture with aluminium oxide whichsupposedly dissolves the aluminium oxide, thereby allowing intimatecontact between aluminium surfaces during the sintering step.

The applicant does not concede that the prior art discussed in thisspecification forms part of the common general knowledge in Australia orelsewhere.

Throughout this specification, the term “comprising” and its grammaticalequivalents are taken to have an inclusive meaning unless the contextindicates otherwise.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a metal injectionmoulding method that allows for the production of articles fromaluminium, aluminium alloys or aluminium matrix composites.

In a first aspect, the present invention provides a method for formingan article by metal injection moulding of aluminium or an aluminiumalloy, the method comprising the steps of:

-   -   forming a mixture containing an aluminium powder or an aluminium        alloy powder or both and optionally ceramic particles, a binder,        and a sintering aid comprising a low melting point metal;    -   injection moulding the mixture;    -   removing the binder; and    -   sintering; wherein sintering is conducted in an atmosphere        containing nitrogen and in the presence of an oxygen getter.

The oxygen-getter may comprise any metal that has a higher affinity foroxygen than aluminium. Some examples of suitable metals for use as theoxygen-getter include the alkali metals, the alkaline earth metals andthe rare earth metals. Where one or more rare earth metals are used asthe oxygen-getter, it is preferred that rare earth metals from thelanthanide series are used.

Magnesium is the preferred metal for use as the oxygen-getter because itis has a high vapour pressure, it is readily available and it isrelatively inexpensive.

In some embodiments, blocks of the oxygen-getter may be positionedaround the article that is being sintered during the sintering step. Inother embodiments, powder of the oxygen-getter may be placed around oron the article that is being sintered during the sintering step. As afurther alternative, the oxygen-getter may be mixed in with thealuminium or aluminium powder alloy, or mixed in with the mixture thatis fed to the injection moulding apparatus.

In a further embodiment, the oxygen getter is present as a component ofan alloy added to the mixture, such as being present in an alloy powderadded to the mixture. For example, powder of an alloy containingaluminium and magnesium (and possibly other components) may be added tothe mixture or incorporated into the mixture. Examples of some alloysthat can be incorporated into the mixture include Al-7.9 wt % Mg andAl-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si.

Without wishing to be bound by theory, the present inventors havepostulated that the oxygen-getter removes any oxygen that may be presentin the atmosphere surrounding the part during the sintering step. Theoxygen-getter may also act to reduce the aluminium oxide that surroundsthe aluminium or aluminium alloy particles. This assists in disruptingthe aluminium oxide layer around the particles, exposing fresh metal,thereby allowing sintering of the aluminium or aluminium alloy particlesto take place.

As mentioned above, magnesium is a suitable oxygen-getter. In additionto being relatively inexpensive, magnesium also has a high vapourpressure. Consequently, during the sintering step (which takes place atelevated temperature), magnesium vapour may surround the article that isbeing sintered.

A sintering aid is added to the mixture prior to injection moulding ofthe mixture. The sintering aid is a low melting point metal. Forexample, the sintering aid may be a metal that has a melting point thatis lower than the melting point of aluminium. Preferably, the sinteringaid comprises a low melting point metal that is insoluble in solidaluminium. Some examples of suitable sintering aids include tin, lead,indium, bismuth and antimony. It has been found that tin is especiallysuitable in assisting in sintering of aluminium and aluminium alloys.Therefore, tin is a preferred sintering aid.

Tin is a preferred sintering aid for use in the present inventionbecause it has been found that tin suppresses the formation of aluminiumnitride during sintering (thereby avoiding formation of excessivealuminium nitride, which might have a detrimental effect on theproperties of the final article) and also changes the surface tension ofmolten aluminium, thereby promoting good distribution of liquidaluminium phase during sintering.

The sintering aid may be added in an amount of up to 10% by weight,based upon the total weight of the metal powder and the sintering aid.Preferably, the sintering aid is present in an amount of from 0.1% to10% by weight, more preferably 0.5% to 3% by weight, even morepreferably about 2% by weight.

Where tin is used as the sintering aid, it may be added in an amount offrom 0.1% to 10%, more suitably from 0.5% to 4%, even more suitably from0.5% to 2.0% by weight of the mixture.

Tin melts at 232° C., which is considerably lower than that of aluminium(660° C.) and there are no intermetallic phrases. Tin is sparinglysoluble in solid aluminium: the maximum solid solubility is less than0.15%. Aluminium is completely miscible in liquid tin and no immiscibleliquids form. Further, the surface tension of liquid tin issignificantly less than that of aluminium and trace amounts of tin havebeen shown by the present inventors to improve the wettingcharacteristics and sintering behaviour of aluminium. For these reasons,tin is an especially preferred sintering aid.

The sintering step is conducted in a nitrogen atmosphere. Withoutwishing to be bound by theory, the present inventors have postulatedthat conducting the sintering step in a nitrogen atmosphere may promotethe formation of aluminium nitride. The present inventors havepostulated that forming aluminium nitride in the sintering step mayassist in disrupting or breaking down the aluminium oxide film thatnormally surrounds the particles of aluminium or aluminium alloy. Theuse of tin as a sintering aid may also assist in controlling theformation of AlN as formation of excessive amounts of AlN duringsintering may cause detriment to the properties of the final article.

If high purity aluminium is being used as a feed powder, the presentinventors have found that conducting sintering of aluminium powder in anitrogen atmosphere can result in the rapid conversion of the aluminiumto aluminium nitride. Due to the rapid rate at which the aluminium canbe converted to aluminium nitride in these circumstances, there is arisk that the entire article may be converted to aluminium nitride.Using tin as a sintering aid acts to limit the formation of excess AlNin such circumstances.

Without wishing to be bound by theory, the present inventors havepostulated that the nitrogen atmosphere disrupts the aluminium oxidefilm on the surface of the aluminium or aluminium alloy particles byforming aluminium nitride. It is further postulated that this disruptionof the aluminium oxide film enables sintering of the aluminium oraluminium alloy particles to occur.

The atmosphere in which the sintering step is conducted may have a lowwater content, for example, it may have a water vapour partial pressureof less than 0.001 kPa. The atmosphere used on the sintering step mayhave a dew point of less than −60° C., more preferably, less than −70°C. Magnesium, when used as an oxygen getter, reacts with oxygen andwater, thereby further lowering the water content of the atmosphere. Itis believed that water vapour is extremely detrimental to the sinteringof aluminium.

The atmosphere is an atmosphere containing nitrogen. The atmosphere maybe predominantly nitrogen. The atmosphere may be 100% nitrogen. Theatmosphere may also include an inert gas. The inert gas may comprise aminor part of the atmosphere. The atmosphere may be essentially free ofoxygen and hydrogen. In this regard, the gas that is supplied as theatmosphere during sintering suitably contains no oxygen or hydrogen.

The binder used in the present invention may be any binder or bindercomposition known to be suitable for use as a binder in metal injectionmoulding. As will be known to persons skilled in the art, the binder istypically an organic component or a mixture of two or more organiccomponents.

The binder desirably includes thermoplastic components that enable thebinder to melt upon application of heat. The binder should also impartsufficient strength to the green body following injection moulding toenable the green body to be handled. Desirably, the binder is able to beremoved from the green body in a manner that retains integrity of thebody during the binder removal process. It is preferable that the binderleaves no residue following removal.

The binder may be made from two or more materials. The two or morematerials that comprise the binder may be selected such that they may besequentially removed from the green body. In this fashion, a controlledremoval of the binder is more easily achieved, thereby facilitatingretention of shape integrity of the body during binder removal. In thisregard, it will be appreciated that if the binder is removed toorapidly, the risk of the body losing its shape integrity is increased.

The binder may be removed by one or more of the known techniques used inmetal injection moulding for removing the binder. For example, thebinder may be removed by dissolution in a solvent, by thermal treatmentto cause the binder to melt, evaporate or decompose, by catalyticremoval of the binder or by wicking.

Two or more binder removal techniques may be used in the binder removalstage. For example, a first step in the binder removal may involvesolvent extraction, followed by thermal removal of the remainder of thebinder.

The person skilled in the art will appreciate that a large range ofbinder materials may be used. Some examples include organic polymerssuch as stearic acid, waxes, paraffin and polyethylene.

Without wishing to be limited in any way, the present inventors haveused a binder comprising stearic acid, palm oil wax and high densitypolyethylene in experimental work relating to the present invention.

The sintering step used in the present invention involves heating thegreen body to a temperature at which the aluminium or aluminium alloysinters to form a dense body. The sintering step suitably involvesheating to a temperature within the range of about 550° C. to about 650°C., more suitably 590° C. to 640° C., most suitably between 610° C. to630° C. The sintering time may vary. Typically, a shorter sintering timemay be used for a higher sintering temperature. Essentially, thesintering time should be long enough to ensure that maximumdensification of the article has occurred. Sintering at temperatures offrom 620° C. to 630° C. for up to two hours has been found to providesatisfactory results. However, both longer sintering times and shortersintering times are encompassed by the present invention.

The heating rates and thermal profile used in the sintering step arenormally closely controlled in metal injection moulding methods toobtain optimum properties in the final article. The person skilled inthe art will readily understand how to determine suitable heating ratesand temperature profiles for use in the sintering step.

The method of the present invention is suitable for use with aluminiummetal and aluminium alloys. Any aluminium alloy can be used in thepresent invention, including aluminium alloys from the 1000 series, 2000series, 3000 series, 4000 series, 5000 series, 6000 series, 7000 seriesand 8000 series.

Ceramic particles can be mixed with the aluminium or aluminium alloypowders to create an aluminium metal matrix composite. The ceramicparticles are used to improve or control the properties of the sinteredarticle. Such properties can include, but are not limited to, wearresistance, stiffness or coefficient of thermal expansion. Anon-exhaustive list of typical ceramic materials include SiC, Al₂O₃,AlN, SiO₂, BN and TiB₂.

The method of the present invention may be carried out in known metalinjection moulding apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photomicrograph of the fracture surface of a test barmade in accordance with an embodiment of the present invention followingdebinding;

FIG. 2 shows photographs of a green body and a sintered body for testbars made in accordance with an embodiment of the present invention;

FIG. 3 is a graph of density and hardness of test pieces made inaccordance with embodiments of the present invention;

FIG. 4 shows graphs of tensile curves of test bars after sintering atvarious conditions;

FIG. 5 shows microstructures of sintered products made in accordancewith an embodiment of the present invention;

FIG. 6 shows a graph of the effect of elemental Mg additions on sintereddensity;

FIG. 7 shows the liquid content as a function of temperature for thealloys listed in FIG. 7;

FIG. 8 shows a graph of the sintered density of AA6061+X % Sn loosepowders as a function of temperature; and

FIG. 9 shows a graph of sintered density as a function of temperaturefor the feedstock mixtures listed in that Figure.

EXAMPLES

A variety of alloy and powder compositions, particle sizes and particleshapes were tested. Spherical AA6061 powders with a D₅₀ of 10 μm andspherical Sn with a particle size <45 82 m were preferred. The metalinjection moulding feedstock consisted of 6061 powder with 2 wt % Sn anda binder system of 3 wt % stearic acid, 52wt % palm oil wax and 45 wt %high density polyethylene. The raw materials were mixed at 165° C. for180 minutes. After granulating, the feedstock was injection moulded intostandard tensile bars using an Arburg moulding machine. Solventdebinding was conducted in hexane at 40° C. for 24 hours. The removal ofthe remaining binder and sintering were combined and conducted in asealed tube furnace. The preferred atmosphere is high purity nitrogengas flowing at 1 litre/min. The thermal profile used in the experimentalwork is shown in Table 1. Magnesium bars were placed around the articleduring sintering.

Tensile testing was conducted on as-sintered material. The extensometergauge length was 25 mm and the crosshead speed was 0.6 mm/min.

Rockwell hardness (HRH) was measured on both top and bottom surfaceswith a ⅛ inch steel ball and 60 kg load.

TABLE 1 Heating profile for debinding and sintering. Step 1 2 3 4 5 6 7Rate 3 0.5 0.5 0.5 0.8 0.5 10 (° C./min) Temp 150 250 375 450 620 550 25(° C.) Hold 0 120 120 60 120 0 End Time (minutes)

Results

FIG. 1 shows a fracture surface of a debound part. The powder morphologyhas not changed from the original.

FIG. 2 shows the injection moulded (green) body and the sintered part.The sintered part is free of defects such as blisters, cracks andwarpage. It also has a good surface finish.

FIG. 3 shows the density and hardness of the test bars under varioussintering conditions. For parts sintered for 1 hour at 620° C. innitrogen, the sintered density was 90.0±0.6% and the hardness was39.1±12.3. The big variation of hardness is possibly due to the highporosity level. When the sintering time was increased to 2 hours, thedensity and hardness increased to 94.9±0.3% and 66.9±2.9, respectively.However, further increasing the sintering temperature to 630° C. did notsignificantly increase the density and hardness. The density in thiscondition was 95.3±0.3% and the hardness was 69.0±0.9.

Typical stress/strain curves of parts sintered at various conditions areplotted in FIG. 4. The part sintered at 620° C. for 2 hours has the bestmechanical properties and recorded a 0.2% yield strength of 58 MPa, atensile strength of 156 MPa and elongation to failure of 8.9%. Thetensile properties of the parts sintered at 630° C. were slightly lowerthan this, despite the higher density. This was possibly due tomicrostructural coarsening at the higher sintering temperature.

For the parts sintered at 620° C. for 1 hour, the low density producedpoor mechanical properties. The tensile strength was 98 MPa and strainwas 1.7%.

FIG. 5 shows the microstructure of a sample after sintering at 620° C.for 2 hours. The optical micrograph shows that the grain size remains atabout the original particle size and is less than 20 μm. Thebackscattered electron image shows the distribution and size of the Snrich phase (white contrast in electron image, black contrast in opticalimage). No obvious pores were visible.

Further Examples

Various percentages of −325 mesh elemental Mg powders or Mg richpre-alloy powders were formulated and mixed into the feedstock. Thefeedstock was then compacted into 25.4 mm diameter discs using a hotmoulding machine. The discs were sintered in nitrogen without Mg blocksbeing present in the furnace. Before sintering disks with pre-alloyedpowders in them, the furnace was run empty at 680° C. for four hours invacuum to remove any Mg residue in the furnace. The parts were loadedinto a steel crucible with a loose lid to minimise the effect of gasflow.

Results

The effect of elemental Mg additions on sintered density is shown inFIG. 6. It was found that 1.0 wt % Mg gave the highest sintered densityof ˜94%. With 0.5 wt % Mg, there is insufficient gettering of oxygen andthe part was distorted due to a porous surface layer. The addition of2.0 wt % elemental Mg powder into the feedstock results in low sintereddensity (˜80%) due to nitriding. Due to safety concerns, addingelemental Mg powder into the feedstock is not preferred. However, addingMg into the feedstock in the form of pre-alloy to powders will overcomesome of the disadvantages of elemental power.

Examples—Adding AIMg Powder to Feedstock

Pre-alloyed powders of composition Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Siand Al-7.9 wt % Mg were obtained from the Aluminium Powder Company. TheAl-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powders have an average particlesize of about 25 μm whilst the Al-7.9 wt % Mg powders have an averageparticle size of about 40 μm. Both have a regular particle shape. TheAl-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si has a solidus temperature around540° C. and it is fully liquefied at 600° C. The Al-7.9 wt % Mg has asolidus temperature around 540° C. and it is fully liquefied at 620° C.FIG. 7 illustrates the liquid content as a function of temperature forthese alloys, as well as alloy AA6061 and for a mixture of AA6061+7.5 wt% Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si.

It has been found that sintering a mixture of AA6061+7.5% Al-2 wt %Cu-9.3 wt % Mg-5.4 wt % Si+2 wt % Sn feedstock at 610° C. for two hoursin nitrogen produces distortion free parts which have a density of ˜97%of theoretical.

Examples—Use of Tin as Sintering Aid

Sn has been used as an effective sintering additive for the pressed orun-compacted aluminium alloys and compacts prepared by rapid prototypingprocesses. The present inventors have demonstrated that Sn plays animportant role in the sintering of tapped loose powders and powderinjection moulded aluminium compacts. However, Sn will remain at thegrain boundaries after sintering since tin is almost insoluble in solidaluminium. Excess amounts of tin will deteriorate the mechanicalproperties, especially the ductility, which is extremely desirable foraluminium alloys prepared from powders.

The debound part (brown part) of powder injection moulded aluminiumcompact only has a relative density about ˜85%. After the polymerbinders are removed, there are open channels connecting to the partsurface in the porous debound part. Tapped loose powders only have arelative density about 40-60%, connected pores may form open channels tothe surface. Significant amount of liquid is needed to seal thesechannels. In previous examples, we found that 4% Sn helps the sinteringof loose compacted pure aluminium powders; addition of 2% Sn enhancedthe sintering of powder injection moulded AA6061 compact. In the presentexamples, we try to minimize the amount of Sn additions whilemaintaining the liquid volume by adding some pre-alloyed aluminiumpowders. The addition of the heavily pre-alloyed powders will also helpincrease alloy content in the sintered part and improve its strength.The decrease of Sn content may help to improve ductility. By such means,the mechanical properties of the alloy system could be further improved.

Elemental Sn (<43 μm) is used as a sintering aid to enhance the liquidphase sintering of fine AA6061 powders (<20 μm) mixed with pre-alloyedAl-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powder (<30 μm). The powders weremixed in a Turbula mixer for 30 minutes according to the formulation ofAA6061+Xwt % Sn+Ywt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si. The mixedpowders were poured into alumina crucibles, tapped and enclosed byaluminium foils. Then they were sintered in a steel tube furnace for 2hours at various temperatures under nitrogen gas flow of 0.5 litre/min.The sintered density was obtained by Archimedes' method and wasconverted into percentage of the theoretical density (TD %) of eachalloy. Polished samples were used for both optical and scanning electronmicroscopy (SEM).

FIG. 8 shows that the sintered density of AA6061+Xwt % Sn loose powdersincreased with higher sintering temperature. There is a density increasefor 2 wt % Sn alloy system at 580° C. and 1 wt % Sn system at 590° C.The addition of Sn obviously enhanced the sintering and a much highersintered density was obtained for alloys with Sn. The sintered densitywas ˜95% or higher for the alloys with 1.0 or 2.0 wt % Sn in thesintering temperature window of 600-630° C. On the contrary, AA6061loose powders without Sn only achieved 83%, 88% and 93% at 610° C., 620°C. and 630° C., respectively.

For liquid phase sintering, the liquid volume is one of the mostcritical factors for the densification and part shape retention. Theliquid volume in the Al—Sn alloy system is controlled by temperature,aluminium alloy composition and the Sn content. FIG. 7 shows the effectof temperature on the liquid volume fraction for the tested alloys. Thisdata was calculated using ThermoCalc. No Sn addition is considered. ForAA6061+xwt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si alloys, thecalculations were based on the final total alloy content. Thepre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powder has a solidus of582° C. and it is fully liquefied at 604° C. So, this alloy will be verydifficult in process control with such a narrow melting window if it issintered alone. However, this early formed liquid with high Mg contentmay scavenge oxygen in the sintering furnace and help to seal the openchannels in the loose powders before serious oxidation occurs, whichusually starts at about 580-600° C.

FIG. 9 shows the sintered density of AA6061+0.5 wt % Sn loose powderswith addition of 0%, 2.5% and 7.5% pre-alloyed Al-2 wt % Cu-9.3 wt %Mg-5.4 wt % Si powders after sintering at various temperatures for 2hours in nitrogen. The sintered density of AA6061+0.5 wt % Sn increasessteadily as function of temperature till 630° C. as liquid volumeincrease. The liquid from the melting of Al-2 wt % Cu-9.3 wt % Mg-5.4 wt% Si powders sharply increased the density at sintering temperature of600° C. for 2.5 wt % addition and 590° C. for 7.5 wt % addition.However, excess liquid soon resulted density decrease at 620° C. afterit peaked at 610° C. for the AA6061+0.5 wt % Sn+7.5 wt % Al-2 wt %Cu-9.3 wt % Mg-5.4 wt % Si alloy system. The density decrease ispossibly due to early formed liquid entrapped gas inside the part. The2.5 wt % addition of pre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Sipowders helped to maintain a density plateau of ˜97% in temperaturerange of 600620° C. The density started to decrease at 630° C.

Those skilled in the art will appreciate that the present invention maybe subject to variations and modifications other than those specificallydescribed. It will be understood that the present invention encompassesall variations and modifications that fall within its spirit and scope.

1. A method for forming an article by metal injection moulding ofaluminium or an aluminium alloy, the method comprising the steps of:forming a mixture containing an aluminium powder or an aluminium alloypowder or both and optionally ceramic particles, a binder, and asintering aid comprising a low melting point metal; injection mouldingthe mixture; removing the binder; and sintering; wherein sintering isconducted in an atmosphere containing nitrogen and in the presence of anoxygen getter.
 2. A method as claimed in claim 1 wherein theoxygen-getter comprises a metal that has a higher affinity for oxygenthan aluminium.
 3. A method as claimed in claim 2 wherein theoxygen-getter is selected from the group consisting of the alkalimetals, the alkaline earth metals and the rare earth metals.
 4. A methodas claimed in claim 3 wherein the oxygen-getter is magnesium.
 5. Amethod as claimed in claim 1 wherein blocks of the oxygen-getter arepositioned around the article that is being sintered during thesintering step or powder of the oxygen-getter is placed around or on thearticle that is being sintered during the sintering step or theoxygen-getter is mixed in with the aluminium or aluminium powder alloy,or mixed in with the mixture that is fed to the injection mouldingapparatus or the oxygen getter is present as a component of an alloyadded to the mixture.
 6. A method as claimed in claim 1 wherein thesintering aid is a metal that has a melting point that is lower than themelting point of aluminium and is insoluble in solid aluminium.
 7. Amethod as claimed in claim 6 wherein the sintering aid comprises tin. 8.A method as claimed in claim 1 wherein the sintering aid is present inan amount of up to 10% by weight, based upon the total weight of themetal powder and the sintering aid.
 9. A method as claimed in claim 8wherein the sintering aid is present in an amount of from 0.1% to 10% byweight.
 10. A method as claimed in claim 8 wherein the sintering aid ispresent in an amount of from 0.5% to 3% by weight.
 11. A method asclaimed in claim 1 wherein the atmosphere in which the sintering step isconducted has a low water content wherein the water vapour partialpressure is less than 0.001 kPa.
 12. A method as claimed in claim 1wherein the binder includes thermoplastic components that enable thebinder to melt upon application of heat.
 13. A method as claimed inclaim 1 wherein the binder is made from two or more materials selectedsuch that they are sequentially removed from the green body.
 14. Amethod as claimed in claim 1 wherein the binder is removed bydissolution in a solvent, by thermal treatment to cause the binder tomelt, evaporate or decompose, by catalytic removal of the binder or bywicking.
 15. A method as claimed in claim 14 wherein two or more binderremoval techniques are used in to remove the binder.
 16. A method asclaimed in claim 1 wherein the binder comprises stearic acid, palm oilwax and high density polyethylene.
 17. A method as claimed in claim 1wherein the sintering step involves heating the green body to atemperature at which the aluminium or aluminium alloy sinters to form adense body.
 18. A method as claimed in claim 17 wherein the temperatureis within the range of about 550° C. to about 650° C.
 19. A method asclaimed in claim 1 wherein the mixture includes ceramic particles andthe ceramic particles are selected from the group consisting of SiC,Al₂O₃, AlN, SiO₂, BN and TiB₂.
 20. A method as claimed in claim 1wherein the atmosphere comprises nitrogen or a mixture of nitrogen andan inert gas.
 21. A method as claimed in claim 1 wherein the atmosphereis essentially free of oxygen or hydrogen.